A case of blackmail threatens to topple the monarchy itself, but soon Sherlock and John discover there is even more to it than that. They find themselves battling international terrorism, rogue CIA agents, and a secret conspiracy involving the British government.

This case, however, will cast a longer shadow on their lives than they could ever imagine, as the great detective begins a long duel of wits with an antagonist as cold, ruthless, and brilliant as himself: Irene Adler.

Jim Moriarty, whose snipers are aimed at Sherlock Holmes and Dr John Watson, and who has stated his intention to kill them both, is interrupted by a phone call. He leaves, having "received a better offer", letting Sherlock and John return to their flat at 221B Baker Street.

Sherlock Holmes solves a number of cases over the next few weeks, turning down several others because they bore him, including a man claiming his aunt's ashes are not hers and two girls not allowed to see their dead grandfather. Sherlock becomes a minor celebrity following John's blogs about his activities. One day, Sherlock's brother Mycroft has the pair brought to Buckingham Palace for a meeting. Mycroft and a Palace official explain that a female member of the royal family has had compromising photographs taken with dominatrix Irene Adler, and that she wishes for them to be retrieved. While Sherlock reviews photos of Adler, who is referred to as "The Woman", she looks at pictures taken of him.

Sherlock and John visit Adler's home, attempting to use deception to get inside. However, Adler is expecting them and after considering possible outfits, she appears fully made-up but completely naked. Sherlock is consequently unable to deduce anything about her. After several rounds of banter between Sherlock and Adler, John sets off the fire alarm and Sherlock is able to determine the location of Adler's safe, where a camera phone containing the compromising photos, as well as other valuable information, is hidden. Several American operatives appear and hold Sherlock, John and Adler at gunpoint, demanding that Sherlock open the safe. Sherlock deduces the password (Adler's measurements) and opens the safe, which is booby-trapped with a handgun and kills one of the assailants. They disarm the rest of them. Sherlock acquires Adler's camera phone, but she attacks him with a drugged syringe and escapes through a window with the phone. During this time, John is in another room. Back at his apartment, Sherlock has a fevered dream in which he sees Irene returning his coat. He wakes to find that his coat has mysteriously reappeared, and Adler has added her number to his phone. She has added a personalised ringtone of a woman sighing erotically, which sounds whenever Sherlock's telephone receives a text message she has sent.

Six months later, whilst celebrating Christmas, Sherlock learns (via text message) that Adler has sent him the camera phone for safekeeping. Sherlock notifies Mycroft that he will shortly find Adler dead, knowing the value Adler put on her phone. Subsequently, the Holmes brothers arrive at St Bart's morgue where Sherlock identifies Adler's mutilated body. Some time later, John is contacted by an unnamed woman who has him brought to the abandoned Battersea Power Station. The contact reveals herself to be Irene, not Mycroft as John expected: she faked her own demise to shake pursuers off her trail. John urges her to reveal herself to Sherlock. She initially refuses, but is persuaded. The two then discuss the nature of their respective relationships with Sherlock; during this, Adler's personalised ringtone is heard from an adjacent corridor, revealing Sherlock's presence and his awareness of Adler's survival.

Back in 221B, Sherlock detects the signs of a break-in and finds the American hitmen from Adler's residence holding Mrs Hudson hostage. Sherlock immediately realises that the landlady has been tortured and plans extreme retaliation. During the stand-off, Sherlock demands two of the gunmen go away, leaving him with their leader. While the man frisks him, Sherlock knocks him out cold, ties him up, and eventually throws him out the window.

Adler kissing Sherlock to encourage him to crack the code.

Later on, Sherlock finds Adler sleeping in his bedroom. Irene reveals that she is still being hunted and asks Sherlock to decipher a code she stole from a Ministry of Defence official. He effortlessly cracks the code, revealing it to be an airline seat allocation number. Irene secretly texts the flight number to her contact, Jim Moriarty. He in turn texts Mycroft Holmes, revealing that he is now aware of the MoD plot to fool a terrorist cell that was attempting to sabotage the flight. Mycroft is visibly shattered by this development.

Adler's attempts to seduce Sherlock are interrupted by government officials who have come to collect him and deliver him to Heathrow airport. En route there, Sherlock remembers Mycroft mentioning Coventry on the phone and reminisces about the allegations that the British government allowed the Coventry Blitz to happen, so as not to alert the Germans that their military codes had been cracked. There, his suspicions that a similar situation is occurring are confirmed by Mycroft on board the airplane, which has been filled with corpses. The government had decided to fly a 'dummy plane', so as not to alert the saboteurs yet still avoid genuine casualties, which also explains the involvement of US agents. However, as Sherlock has unwittingly helped Irene, and, by extension, Moriarty, crack the code, the scheme is foiled.

Afterwards, the Holmes brothers and Adler sit down, while Adler reveals a list of demands, including protective measures for herself, against the release of further confidential material. Mycroft is helpless; however, at the last moment, Sherlock confronts Adler, deducing the password of her camera phone. Despite Adler's cold insistence that Sherlock meant nothing to her, taunting his lack of sexual or romantic experience, he has already concluded she is lying. He points out her dilated pupils and her elevated pulse during moments of intimacy – both indicators of her own attraction to him. He types S-H-E-R into the phone, which reads "I AM S-H-E-R LOCKED". Passing it to his brother, Sherlock walks out, ignoring Adler's pleas for protection. Without the insurance of the information she held, she is unlikely to be able to outrun her enemies.

Some months later, Mycroft informs John that Adler has been beheaded by a terrorist cell in Karachi, but asks him to tell Sherlock instead that she has entered a US witness protection program. Sherlock appears to accept this and asks John to give him Adler's phone. John leaves, while Sherlock re-reads her multiple flirtatious text messages to him. The scene changes to a flashback of Adler's "execution": Sherlock had infiltrated the cell, disguised as her executioner, and rescued her with seconds to spare.

Vatican Cameos is a codeword used between Sherlock Holmes and Dr. John Watson. Vatican Cameos was used again in "The Sign of Three" when Sherlock says it during his best man speech to warn John about an impending murder. When Mary asks what it means, John tells her it means somebody is going to die.

The phrase is similar to the 'Headless Nun' ploy from the "Unaired Pilot", which appears to mean 'Throw me out (of the restaurant) while I pretend to be drunk'. Judging from Angelo's reaction, this had happened before.

"What's that?" Sherlock asks. "It is the title..." John replies. "What does it need a title for?" The title was, "The Geek Interpreter," referencing Doyle's "The Adventure of the Greek Interpreter".

One of John's blog posts is titled "Sherlock Holmes Baffled", the same title as a 1900 silent film, the first depiction of Holmes on film.

According to John, Sherlock enumerates 240 different types of tobacco ash on his website. This is a reference to The Sign of the Four, in which Holmes tells Watson that he has written a monograph in which he enumerates 140 forms of tobacco.

Sherlock uses a deerstalker cap in an attempt to disguise himself from the media, a reference to the classic image of him created by illustrator Sidney Paget.[citation needed]

John reveals his middle name to be Hamish, in accordance with popular supposition.

Moriarty's text message to Mycroft of "Dear me, Mr Holmes. Dear me." is his note to Holmes in the epilogue of The Valley of Fear.

As Sherlock opens Irene Adler's safe, he says "Vatican cameos", a reference to an unknown case Sir Arthur Conan Doyle briefly mentioned during The Hound of the Baskervilles. It says; "I was exceedingly preoccupied by that little affair of the Vatican cameos, and in my anxiety to oblige the Pope I lost touch with several interesting English cases"

As, once more, an 'update' of a literary Holmesian trait, Sherlock's encyclopaedic knowledge of London trains appears here to have been transmogrified into total recall of, at least, all flights in and out of Heathrow, given the precision of his near-instantaneous analysis of Irene's purloined email.

Near the end of the episode, one of Sherlock and Irene's texts reads "BBC1 now. You'll laugh", indicating the time that Sherlock was on when the episode was first aired.

This episode covers the largest amount of time so far (roughly 11 months). It picks up on the last episode (April 1), and quickly moves through the entire summer. They meet Irene in September, and the episode continues through Christmas and ends in March of the following year. According to John's blog, the next case, "The Hounds of Baskerville", happens before 16 March.[1]

It's easy to miss a visual joke. When a woman meets John to take him to Irene Adler in a black car, he thinks it's Mycroft and says, "You know, Mycroft could just phone me, if he didn’t have this bloody stupid power complex.".The scene cuts to the empty shell of Battersea Power Station, which is in fact the biggest power complex in the neighbourhood.

According to the DVD commentary, Producer Sue Vertue was at a funeral when someone's cellphone went off. It was, most inappropriately, playing 'Staying Alive' by the Bee Gees. When her husband, Steven Moffat, heard this he decided it was the perfect ringtone for Moriarty.

The beginning scene was shot 18 months after the earlier part of the scene in "The Great Game". On closer observation, it can be seen that Sherlock's face is a bit fuller and Watson is wearing a wig.

John revels his middle name to be "Hamish". In the canon, the preface of the collection His Last Bow is signed 'John H. Watson, M.D', and in "The Problem of Thor Bridge", Watson says that his dispatch box is labeled 'John H. Watson, M.D'. His wife Mary Watson calls him 'James' in "The Man with the Twisted Lip"; Dorothy L. Sayers speculates that Mary may be referring to his middle name Hamish (an Anglicisation of 'Sheumais', the vocative form of 'Seumas', the Scottish Gaelic for James)[2], though Doyle himself never addresses this beyond including the initial.

"The Speckled Blonde", "The Geek Interpreter," and "The Aluminium Crutch" are documented on John's blog. [3][4][5]

The deleted "Analysis of Tobacco Ash" can be seen in the case files of Sherlock's website. [6]

John's blog counter has frozen at 1895. This is an allusion to a poem loved by the show's creators, "221B" by Vincent Starrett, one of the first Sherlockians.[citation needed] The last two lines are: "Here, though the world explode, these two survive / and it is always eighteen ninety-five." 1895 was the year in which Sherlock Holmes was at his prime, according to the original stories by Conan Doyle.

According to the DVD commentary, Benedict Cumberbatch ended up with nicotine poisoning after doing the smoking in the morgue scene about 20 times.

When Mycroft is pouring tea from a teapot, he says, "I’ll be mother." It refers to the old-fashioned superstition that only one person in the household – usually the mother of the family – should pour the tea.[8]

Sherlock steals a glass ashtray from Buckingham Palace. On John's blog about this visit, Mrs Hudson comments, "Is that where this ashtray came from?!", to which John responds "Remind me never to ask you for an alibi, Mrs H.".[9]

According to Sherlock, she was born in the 80s. This means (at the time of airing), her character was between 22–31 years old.

A woman named Siobhan Whelan commented on John's blog "The Speckled Blonde" that "My husband died last week and they wouldn't let me see the body and I'm sure they're hiding something." and asked for Sherlock's help[10], which Sherlock dismissed on "Hat-Man and Robin" blog.[11] Another hint of "The Coventry conundrum" Sherlock missed.

According to the ticket stub from the dead passenger, his name was John Coniston and he was flying on flight number 007 from Flyaway Airways.

This episode provoked controversy due to Irene Adler's nude scenes. Purist and feminist viewers also did not approve of the portrayal of the character. However, the creators dismissed these allegations.

The script for this episode was awarded the Edgar Allan Poe Award.

In the credits of Sherlock on Masterpiece Mystery on PBS, some of the letters were highlighted red. Letters from this episode spells out T-H-E-W-O-M-A-N.

In the opening poolside scene, John's jacket containing the explosives is constantly seen in different positions.

At Buckingham Palace, Watson has just sat down next to Sherlock. There is a red pillow between them on the couch. It moves, sometime sitting straight and sometimes tilted, between shots throughout the scene.

When John comes in from the rain and tells Sherlock about Irene, the damp patches on his jacket come and go.

The morning after the Christmas party, John is shown putting on his jacket while speaking to Mrs Hudson, then putting it on again as Mrs Hudson leaves the room.

Near the end, when Mycroft is talking to Watson, he folds his hands and places his head on them. However, in the next shot, his hands are a couple inches away from his face.

When Sherlock and Mycroft go to identify a body at St. Barts Hospital, they enter a door marked "Morgue". Like most British hospitals, Barts has a Mortuary, not a Morgue which is a mainly American word.

In the first paper - with the headline "Hat-man and Robin" - there are two paragraphs of text, and then parts of that text is repeated to fill in the page. Also, on one of the later editions we see, words from that first newspaper are repeated almost verbatim.

On a scene, reflection of crew is visible moving in the mirror on the wall behind and to the right of Mycroft.